1. Field of the Invention
The present invention relates to a method of producing carbon fibers which mean both oxidized fibers and carbonized fibers in this specification and claims, and more specifically to an efficient treatment process of the oxidizing atmosphere discharged from the heat-treating device for producing carbon fibers.
2. Description of the Prior Art
Carbon fibers are usually produced by heat-treatment of acrylic fibers, pitch fibers or polyvinylalcoholic fibers under conditions of temperature and atmosphere suitable for the respective fibers. We will take acrylic carbon fibers as an example hereunder. First, acrylic fibers are heated and fired at 200.degree.-280.degree. C. in an oxidizing gas (for example in air) to make them so-called "oxidized fibers" and subsequently they are carbonized at 800.degree.-2,800.degree. C. in an inert gas (for example in nitrogen gas), thereby producing carbon fibers.
With respect to such heat treatment process, among others, in the case of the process of heating acrylic fibers in an oxidizing gas at 200.degree.-280.degree. C., decomposed products of precursors or oil adhered to precursors such as HCN, NH.sub.3, CO or tar-like substance are released into the atmospheric gas and accumulated to a high concentration. The fibers treated in such an atmosphere are liable to be involved in troubles such as destructions of fiber surface or remarkable deterioration of physical properties of the produced carbon fibers due to voids formed therein.
Well-known countermeasures for these troubles in the conventional heat treatment are: (1) a part of the atmosphere containing said decomposed products (hereinafter referred to as "exhaust gas", which normally accounts for about 20% of the total volume of the atmosphere) is discharged out of the system, while a fresh atmosphere heated to a specified temperature is replenished; or (2) the exhaust gas is totally decomposed by means of an oxidizing catalyst and recycled for use (Japanese Patent Application Laid-open No. SHO 57-25417), but all of these measures have a drawback of suffering a heavy loss of heat.
In the case of above (1) in which the exhaust gas of 200.degree.-280.degree. C. is discharged and replaced with an equivalent volume of the atmosphere which is heated to the same temperature of 200.degree.-280.degree. C., the loss of thermal energy attendant on said discharge and replacement of the atmosphere is naturally heavy. In the case of (2) above in which the decomposed products contained in the exhaust gas are treated by a catalyst in the course of circulation of the exhaust gas and then used again, the loss in thermal energy may be substantially less than in the case of (1) provided the loss is limited to a loss in the circulating channel. According to the publication in the official gazette of said Japanese Patent Application Laid-open No. SHO 57-25417, however, in said catalyst treatment, the exhaust gas has to be heated to 200.degree.-400.degree. C. for the purpose of enhancing the catalyst action. Investigation by the present inventors indicates that for the purpose of (2) being applied to on an industrial scale, the exhaust gas has to be heated to at least 280.degree. C., otherwise the catalytic action would not be satisfactory. Thus in the case of (2), the exhaust gas must be heated to at least 280.degree. C. to enhance the catalytic action and then it must be cooled to a suitable atmospheric temperature after catalyst treatment, thereby making the heat loss substantial, because the total volume of exhaust gas is subjected to catalyst treatment.